The processes by which we hear auditory events in most everyday acoustic environments that produce ndirect sound (e.g. echoes, reflections, reverberation) are complex and not well understood. Under certain circumstances, indirect sound can facilitate speech communication and certain aspects of sound localization, n other circumstances, however, indirect sound can produce deficits in these and other abilities that can be particularly large for individuals with hearing impairment. Recent results have demonstrated that some aspects of indirect sound processing appear to be affected by previous exposure to the acoustic environment, which suggests a form of perceptual adaptation. Although these adaptation effects can be substantial for situations with a single echo, the effects have not been evaluated in more realistic acoustic environments with complex patterns of indirect sound resulting from multiple echoes and reverberation. The ong-term goal of this project is a complete understanding of the mechanisms and the potentially adaptive processes that subserve auditory localization and communication in everyday acoustic environments with complex patterns of indirect sound and the potential impact of hearing loss on these processes. This goal will be addressed by the following Specific Aims: Specific Aim 1: Demonstrate and quantify environment- adaptation effects on sound localization and speech intelligibility for a single realistic acoustical environment configuration. Specific Aim 2: Determine the sensitivity of environment-adaptation effects to particular environments and source/listener configurations. Specific Aim 3: Quantify the time course of environment- adaptation effects. Specific Aim 4: Quantify the effects of source or listener motion on environment- adaptation effects. The research proposed to address these aims uses state-of-the-art virtual acoustic environment simulation techniques for auditory stimulus control that enable testing of listening situations that would not be possible in real environments. Knowledge gained from these studies will lead to an improved understanding of a significant public health problem: the impairment of communication and localization in acoustically reflective or reverberant environments resulting from hearing loss. This knowledge may facilitate improvements in both assistive technologies (signal processing algorithms within hearing aids and cochlear implants) and the acoustical design of listening environments. KEYWORDS: Sound Localization, Spatial Hearing, Speech Intelligibility, Room Acoustics, Adaptation, Virtual Auditory Space.